Many oil formations are known to exist in which the oil or liquid petroleum occupies the pore spaces in the upper portion of a permeable formation, and the water occupies the pore spaces in the lower portion of the same or a contiguous permeable formation. Water encountered in such aquifers underlying oil containing formations is ordinarily brine, having a specific gravity greater than 1 and frequently in the range of from 1.01 to 1.3, whereas the specific gravity of oil is about 0.8. Many crude oils are known which have higher specific gravities, but almost invariably the specific gravity of petroleum encountered in subterranean formations is less than the specific gravity of brines ordinarily associated with the petroleum formations. It is common practice to drill a producing well into such an oil formation to a point near the bottom of the oil-saturated interval, then to plug off the bottom of the well and to establish perforations through the well casing into the oil saturated formation through which fluid petroleum may flow, which permits it to be pumped or otherwise transported to the surface of the earth through the well. In many formations, a number of conditions coexist which results in the establishment of a phenomena known as bottom water coning, in which the oil-water interface is reformed from a previous essentially horizontal configuration into a cone rising along the walls of the oil well. This cone-shaped water saturated interval is formed as a result of pressure differentials created in the formation, which causes water to rise into the previously oil-saturated portion of the permeable formation despite the specific gravity difference between oil and water which would ordinarily maintain the water at a lower level. This problem frequently becomes so acute that in some instances the water-oil ratio rises to such a high value that further production of oil from the well is not commercially feasible. If the well is shut in and the pressure differentials created in the formation during the period of fluid production are allowed to equalize, some levelling of the oil-water interface occurs slowly, although the problem quickly reoccurs when production of oil from the well is resumed.
Many methods have been proposed in the prior art for treating a well afflicted with bottom water coning so as to reduce or eliminate the tendency for water to rise along the walls of a producing well resulting in the production of an ever increasing fraction of water from the well. Two methods have been disclosed by one of the present Applicants in U.S. Pat. No. 4,475,593 and in 4,662,449. U.S. Pat. No. 4,475,593 discloses a method for profile control in subterranean formations by the creation of a barrier at some desired distance remote from the well bore. The unique process of these two patents can be utilized to selectively exclude water from water producing intervals interspersed with the oil producing intervals in certain types of formation, specifically formations whose mineral matrix is mainly large unconsolidated sand-grains but it has not always been entirely satisfactory when used for treating a well completed in a limestone or dolomite formation. It is believed that the reason that the method has been successful when used is unconsolidated sands but has not always been entirely satisfactory when applied to limestone or dolomite oil formation is related to the differences in response of these formations to the acid wash pretreatment used in those cases. When the acid wash contacts large unconsolidated and particles in, for example, viscous oil formations whose mineral component is predominately unconsolidated sand, the extractable polyvalent metal ions which cause demulsification of the resin emulsion injected subsequently are quickly removed from the sand grain surface, leaving a zone of sand which is free of extractable metal ions to form a zone immediately adjacent to the well in which plugging will not occur. In the next step, the injected resin emulsion passes through the zone treated in the acid prewash step, and encounters no extractable metal ions. Since no polyvalent metal ions are contacted in this zone, no plugging occurs. When the resin emulsion passes beyond the acid prewashed zone, polyvalent metal ions are encountered, which causes precipitation of the resin from the resin emulsion, which quickly plugs the flow channels, forming the desired barrier, while leaving a zone between the barrier and the well which is unaffected.
By contrast, when the acid prewash is injected into a formation whose mineral constituent is highly consolidated limestone or dolomite, a different result is obtained. In consolidated sandstones, the metal ions, while present, are often not immediately accessible on the surface of the mineral matrix. The extractable polyvalent metal ions which cause resin precipitation from the resin emulsion treating fluid are distributed more uniformly throughout the consolidated mineral matrix. While the ions on the surface of the formations are removed by the acid wash, the surface left behind is not free of such extractable ions and so when the resin emulsion is injected, additional ions are removed, resulting in precipitation of the resin prematurely, thus reducing the permeability of the formation near the wellbore where no permeability reduction is desired.
Although many of the prior art methods proposed for treating wells to form a barrier in the formation for selective water exclusion or to cure bottom water coning have been at least partly successful in some applications, the problem still remain which are frequently encountered in consolidated formations. Accordingly, there is a substantial unfulfilled need for a method which is especially suitable for treating wells for preventing steam override or for selective water exclusion or to reduce bottom water coning in consolidated oil formations including limestone and dolomite.